Light modulating device for use in television receivers



July 21, 1942. P. NAG? HAL 2,290,569

LIGHT MODULATING DEVICE FOR USE IN TELEVISION RECEIVERS Filed Sept. 4, 1940 3 Sheets-Sheet J.-

July 21,1.942.' P. NAGY ETAL 2,290,569

LIGHT MODULATING DEVICE FOR USE IN TELEVISION RECEIVERS Filed Sept. 4, 1940 3 Sheets-Sheet 2 l\\ A r i lli w/a r w Z 22F220 4 July 21, 1942. P. NAGY HAL 2,290,569

LIGHT MODULATIEG DEVICE FOR USE IN TELEVISION RECEIVERS Filed Sept. 4, 1940 3 Sheets-Sheet 3 P 42 A? 2 JL T 4 LIGHT MODULATING DEVICE FOR USE IN TELEVISION RECEIVERS Paul Nagy, London, and Marcus James Goddard,

- Newbury, England 4, 1940, Serial No. 355,292

Application September In Great Britain 11 Claims.

This invention relates to light modulating devices of the general kind forming the subject of our United States application S. No. 319,858 filed February 20, 1940. Such devices comprise elements of a transparent medium which are capable of substantially independent mechanical oscillations, a device, such as a piezo-electric crystal, associated with each element for producing said oscillations, and an electron discharge tube for successively exciting, and for controlling the excitation of, the oscillation producing devices. This excitation is efiected by means of a cathode ray beam of the electron discharge tube, which makes contact with a contact row of electrically conductive strips associated with the oscillation producing device, each strip corresponding to one element of the transparent medium, and in this way conveys to said strips electrical impulses proportional to applied electrical signals such as the received picture signals of a television transmismm.

In our United States application S. No. 319,858

it is stated that only one electrical. impulse may be applied to each strip of the contact row in any one traverse of the cathode ray beam,or alternatively a number of impulses are applied in succession to each strip, the frequency of said impulses preferably being near the natural resonance frequency of the oscillation producing device. If the former method is adopted, the current reaching the strips of the contact row from the cathode ray beam is modulated with the received electrical signals, e. g. picture signals, only; this may be efiected by applying said received signals either to a grid controlling the current in the cathode ray beam or to a pair of deflection plates which deflect the cathode ray beam across the contact row. If the latter method is adopted, the current reaching the strips of the contact row from the cathode ray beam is modulated both with the received electrical signals and with an excitation carrier signal of the frequency at which the impulses are to be applied to each strip of the contact row. The received electrical signals may be applied to a grid controlling the current intensity of the cathode ray beam, and the carrier signal to a pair of deflection plates which deflect the cathode ray beam across the contact row, or vice versa. Alternatively the received signals may be applied to one control grid and the carrier signal to another, or the received signals to one pair of deflection plates and the carrier signal to another pair acting in the same direction as the first pair. Alternatively the carrier signal may be February 23, 1939 modulated with the received signals, and may then be applied either to a controlfgrid" or to a pair of deflection plates. The use of deflection plates only may be advantageou when modulation of the current of the cathode ray beam by control grids seriously affects the definition of the electron image which is formed by the cathode ray beam on the strips of the contact row.

In the specification of our above-mentioned United States application S. No. 319,858 it is further stated that the impulses applied in succession to each strip of the contact row do not constitute a pure A. C. signal, but take the form of a succession of pulses. These pulses charge electrically the strips of the contact row, and a leakage resistance is associated with each strip, said resistance being of such a value that the charge on the strip substantially leaks away after being established by the cathode ray beam by the time the cathode ray beam returns to recharge that strip.

According to the present invention means are provided whereby the signal applied to each strip of the contact row constitutes a pure A. C. signal or a rectified A. C..signal. The strips do not therefore acquire a resultant electrical charge due to the impact of the cathode ray beam, and

' therefore no leakage resistances associated with the strips are necessary.

The invention will be more particularly described with reference to th accompanying drawings, wherein:

Figure 1 is a diagrammatic section of one form of the device according to the invention, in a plane normal to the direction in which the elements of the light modulating cell are successively actuated by the cathode ray beam.

Figure 2 shows an alternative form of part of the device shown in Figure 1.

Figures 3 to 6 show alternative forms of the device.

Figure 3 is a diagrammatic section of one of said alternative'forms of the device in a plane normal to the direction in which the elements of the light modulatin cell are successively actuated by the cathode ray beam.

Figure 4 is a diagrammatic section of the device shown in Figure 3, the section being taken at right angles to the section of Figure 3.

Figure 5 is a diagrammatic section of a further alternative form of the device in a plane normal to the direction in which the elements of the light modulating cell are successively actuated by the cathode ray beam.

Figure 6 is a diagrammatic section of the device shown in Figure 5, the section being taken at right angles to the section of Figure 5.

Referring now to Figure 1, It! represents the envelope of the electron discharge device which actuates the light modulating cell II. The detailed construction of said light modulating cell is not shown in the diagram. The cathode ray beam is produced by any suitable electrode system, and in Figure 1 the electrode system shown comprises a cathode l6, a control "grid" 11-, an anode l8, and an electromagnetic focusing coil l9 which focuses an electron image on the contact row, of which one strip is shown at [3. The cathode ray beam is caused to scan the strips of the contact row by suitable means, e. g. by applying saw-tooth oscillations to a pair of electrostatic deflection plates. One such plate is shown at 20 in Figure 1. If the light modulating cell is situated outside the envelope of the electron discharge device, as shown in the diagram, each strip of the contact row is connected to another strip, outside the electron discharge device, which excites oscillations in the oscillation producing device of the light modulating cell. One of the strips outside the electron discharge device is shown at I2 in Figure 1. Preferably an electrode 22 is situated close to the contact row to collect secondary electrons emitted by the strips constituting the contact row. This prevents secondary electrons from passing from one strip of the contact row to another, which would effectively reduce the definition of the electron image on the contact row, and also reduces space-charge near the contact row. The electrode 22 may also perform other functions which will be described later. It is preferably constructed in the form of a grid whose members consist of wires or strips running parallel to the direct-ion of scanning. The electrostatic fields set up by the members of the grid do not then cause defocusing of the electron image in the direction of scanning, which is the direction in which an exact focus is required.

Each strip l3 of the contact row consists effectively of two electrically connected sections 14 and I5. Each of these sections is so constructed that, if the cathode ray beam continues to impinge upon it, it acquires a particular characteristic potential. The characteristic potentials of the two sections are 'difierent from one another, but the characteristic potential of all the corresponding sections of the strips of the contact row is the same.

Carrier signals are applied to a pair of electrostatic deflection plates 2|, thereby causing the cathode ray beam to oscillate from one section of the contact row to the other. This oscillation causes the potential of the strip of the contact row upon which the cathode ray beam is momentarily falling to vary between the characteristic potentials of the two sections of said strip, thereby producing a pure A. 0. signal on the strip.

One method whereby the respective sections of the strips may. be made to exhibit different characteristic potentials utilises the variation of secondary electron emission coefiicients of different substances with the velocity with which primary electrons are incident on surfaces of said substances. The secondary emission coeflicient of all substances, so far as is known, increases with increase of velocity of primary electrons up to a maximum at some definite velocity of the primary electrons, and then decreases. For most substances the maximum secondary emission coefficient is greater than unity, but the secondary emission coeflicient becomes less than unity when the velocity of the primary electrons is made sufficiently large.

The velocity at which primary electrons from a cathode ray beam strike a contact surface is determined by the difference of potential between the cathode producing the cathode ray beam and the contact surface. Thus if the cathode is maintained at some constant potential, the velocity of the primary electrons striking the contact surface is determined by the potential of the contact surface. Thus over a certain range of potentials below a critical potential the secondary emission coefiicient of the 'contact surface is greater than unity, while over the whole range of potentials above that critical potential the secondary emission coefficient is less than unity.

When means are provided for collecting all the secondary electrons from the contact surface, e. g. by placing near said surface a grid whose potential is always more positive than that of the surface, then if the potential of the contact surface is within a certain range of potentials below the critica potential, electrons leave the surface in larger numbers than they arrive, and the potential of the surface rises towards the critical value, but if the potential of the surface is above the critical value, electrons leave the surface in smaller numbers than they arrive, and the potential of the surface falls towards the critical value. In either case the po-= tential of the contact surface ultimately reaches the critical value, which constitutes the equilibrium potential at which the number of secondary electrons leaving the surface is equal to the number of primary electrons reaching it.

The critical equilibrium potential is different for different substances. Consequently if the two sections l4 and I5 of the strips l3 (Figure l) are composed of different substances, and the electrode 22 is maintained at a potential higher than that ever acquired by the strips l3 of the contact row, so that said electrode collects all the secondary electrons emitted from the contact row, then the two characteristic potentials of the sections of the st rips l3 are thereby provided.

As the strips I3 need to be electrically conductive, they are conveniently formed of metallic strips. The two sections may consist of strips of two different metals, deposited on or held in position by an insulating substance, and slightly overlapping to produce electrical contact between them. Alternatively one metal may be used throughout to form the basic substance of each strip, and another metal may be deposited on one part of the strip; or, if desired, both metals forming the contact surface may be deposited on opposite ends of a basic strip of a third metal.

The utilisation of different secondary emission characteristic in the way described allows of the use of several slightly different forms of modulation. One form utilises the fact that, if the electron image of the cathode ray beam falls partly on one contact material and partly on the other; the rate at which secondary electron: leave the strip of the contact row with whict the cathode ray beam is making contact is intermediate between what it would be if the cathode ray beam fell wholly on the one or wholly on the other contact material. The equilibrium potential of the strip thus lies at a value intermediate. the critical potentials of the two contact materials. At zero modulation the cathode ray beam is made to fall substantially symmetrically on the two contact. materials, and the potential or the strip of the contact row under excitation then acquires a potentialsubstantially mid-way between the critical potentials of the two contact materials. Modulation is introduced by causing the cathode ray beam to oscillate at carrier frequency about said symmetrical position, the level of modulation being proportional to the amplitude of the oscillation. The modulation is introduced by modulating the carrier signal applied to the electrostatic deflection plates 2i (Figure 1) with the modulating signals, that is, the electrical signals in accordance with which it is desired to modulate the light passing through the cell.

This form of modulation is a pure A. C. modulation. Alternatively a rectified A. C. modulation may be employed. At zero modulation the cathode ray beam then falls entirely on one contact material, and modulation is produced by deflecting the beam at carrier frequency on to the other contact material, full modulation being produced when the beam at maximum deflection lies just wholly on said other contact material. This rectified A. C. modulation may be introduced by applying a corresponding A. C. modulation to the deflection plates 2|, the carrier signal applied to said deflection plates being modulated with the modulating signals. Alternatively a constant carrier signal may be applied to the deflection plates 2| and the modulating signals applied to the auxiliary electrostatic deflection plates 23. In this case the cathode ray beam oscillates continuously at carrier frequency over the strips of the contact row, but when unmodulated it lies always on one of the contact materials. Modulation brings it during part of the cycle of each oscillation on to the other contact material. Full modulation is reached when the oscillation takes place between positions lying just wholly on one contact material and positions lying just wholly on the other contact material.

In the methods of modulation just described, it is necessary that the beam current of the oathode ray beam should be suflicient to charge the strips of the contact row, with their associated connections to the light modulating cell, to the equilibrium potential in the time allowed for this at the frequency of the carrier oscillation. The current may be in excess of the required value, but this is wasteful; if it is less than the required value, the charge acquired by the strips is in sufilcient to bring the potential to the equilibrium value when the deflection is rapid, and a lower level of modulation is produced.

This fact is utilised in an alternative method of modulation. A carrier signal of constant amplitude is applied to the deflection plates 2| (Figure 1). The cathode ray beam is thereby made to oscillate, preferably symmetrically, across the strips I3 of the contact row, preferably between positions lying wholly on one contact material and positions lying wholly on the other contact material. Modulation is produced by applying the modulating signals to the control-grid ll of the electron discharge device. At zero modulation, no current flows in the cathode ray beam, and so no signals are produced on the contact row. At full modulation the current in the cathode ray beam may be just suflicient to librium potentials, so that a potential alternating between the equilibrium potentials at the maximum deflections of the cathode ray beam is produced on the contact row. Advantageously, however, the current in the cathode ray beam at full modulation is less than this, so that the equilibrium potentials at the maximum deflections of the cathode ray beam are never reached, and on the contact row an alternating potential is produced having some definite amplitude less than the diilerence between said equilibrium potentials at the maximum deflections. In fact the sections of the strips of the contact row need not possess critical potentials; this fact will be discussed in more detail later. At intermediate levels of modulation the current in the cathode ray beam is insuilicient to charge the strips of the contact row to the same extent, and an alternating potential of corresponding intermediate amplitude is produced on the contact row.

An alternative form of construction of the strips of the contact row may be adopted. Each strip is divided into sect-ions, not in the direction at right angles to scanning as shown in Figure 1, but in the direction of scanning itself. Also the number of sections into which each strip is divided may be more than two, but only two materials are employed to constitute the contact materials, alternate sections of each strip being composed of one contact material, and the intermediate sections of the other contact material. The electron image of the cathode ray beam must be sufliciently narrow to be capable of falling substantially entirely on one section of any strip of the contact row; it may, however, be in the form of a series of such narrow lines, of

number equal to the number of sections of one falls alternately on charge the strips of the contact row to the equimaterial used in each strip of the contact row, and of separation equal to the separation between adjacent sections of one material. The latter form of electron image may be produced by a multiple cathode ray beam issuing from a multiple cathode. No deflection plates such as those shown at 2| in Figure l are employed to introduce the carrier signal, but the cathode ray beam, in scanning the strips of the contact row,

the sections of difierent materials, and, at full modulation charges each to the critical equilibrium potential of that material of which it is composed, Or to some other potential short of equilibrium as described with reference to the last described method of modulation, thus producing an alternating potential on each strip of the contact row. The number of sections to each strip of the contact row is so chosen that, at the velocity at which the cathode ray beam scans the strips, the frequency of the alternating potential produced is the carrier frequency required to actuate the oscillation producing devices of the light modulating cell.

Modulation may in this case be produced by applying the modulating signals to the control grid of the electron discharge device, or alternatively it may be produced by applying said signals to a pair of electrostatic deflection plates, such as those shown at 23 in Figure 1, thereby deflecting the cathode ray beam laterally on to and off the contact row, thus modulating the signal applied to the contact row. This is turn modulates the light in the manner described in our specification referred to above.

The use of the methods of modulation of the carrier depending on control of the rate of charging of the strips, for example by variation of the beam current of the cathode ray beam, presents four advantages, which have not been fully described above, as compared with the use of the methods depending on the movement of the cathode ray beam to different parts of the contact row where corresponding equilibrium potentials are established. Firstly, in methods where the beam current is not modulated, said beam current is constant at all levels of modulation, but in methods where the beam current is modulated, said beam current is reduced at low levels of modulation, with a corresponding reduction in the power consumption of the device. Secondly, when a strip of the contact row is near its equilibrium potential, the rate at which electrons leave it is only a little greater or a little less than the rate at which they reach it. Consequently the rate of change in the Charge on the strip is small compared with the rate at which charge is arriving from the cathode ray beam, and charging is therefore ineflicient. When the modulation is produced by control of the cathode ray beam current, the strips of the contact row do not usually approach their equilibrium potentials, and charging is therefore more efficient. Thirdly, when modulation is produced by control of the cathode ray beam current, the strips of the contact row need not possess equilibrium potentials corresponding to definite parts of said strips, and hence the two sections of which said strips are composed need not possess critical equilibrium potentials. All that is necessary is that part of the strips should emit secondary electrons in greater numbers, and part in lesser numbers, than it receives primary electrons. Preferably, in fact, part should emit twice as many secondary electrons as it receives primary electrons, and part should emit as few secondary electrons as possible, at all potentials which it acquires during operation. Fourthly, the critical potentials are almost invariably several hundreds, if not thousands, of volts positive with respect to the cathode of the electron discharge device, and

hence, when modulation is effected by the establishment of different equilibrium potentials, a difference of potential of this order always exists between said cathode and the strips of the contact row. The power consumption of the device is proportional to this difference of potential. When the necessity for the existence of equilibrium potentials is obviated by the use of modulation depending on the control of the cathode ray beam current, the device may be operated with a considerably smaller difierence of potential between cathode and contact row, with a correspondingly smaller power consumption.

The third of these four facts makes possible the use of an alternative form of construction for the sections of the contact row, which provides greater efficiency of charging. This construction is shown in Figure 2.

Figure 2 shows a portion of the envelope l of the electron discharge device shown in Figure 1, with the light modulating cell H, the strip [2 associated with the light modulating cell, and the strip l3 of the contact row, which is divided into sections l4 and I5. Adjacent the section M of the contact row there is provided an electrode 24, which is maintained at a potential slightly (i. e. of the order of 100 volts) positive with respect to the cathode l6 (Figure 1) of the electron discharge device. The electrode 22 (Figure 2) which is maintained at a potential approximately equal to, or higher than any potential which any part of the contact row needs to acquire during operation, is provided adjacent to the section 15 of the contact row and the electrode 24. It serves to collect secondary electrons from the section 15 of the contact row and to screen the remainder of the electron discharge device from the potentials of the contact row and the electrode 24. The section ii of the contact row is composed of or coated with a substance of high secondary electron emission coefficient. The section l4 of the contact row may be of any suitable electrically conductive material, but for convenience of manufacture it may advantageously be of the same material as the section 15, and in any case it should be of a material having a secondary electron emission coeflicient greater than unity at the minimum limitng value as defined hereinafter, of the potential of the strip.

The potential of the electrode 22'is, in the present form of the device, fixed at a value less than the critical potential of the section l5 of the contact row already defined with reference to Figure 1. When the cathode ray beam strikes the section l5 of a strip of the contact row, more electrons leave that strip than arrive, and the po-- tential of the strip rises due to the positive charging of the strip. If the cathode ray beam remains in contact with said section [5, this process continues until the potential of the strip reaches a value near the potential of the electrode 22. Some of the secondary electrons then return to the strip instead of going to the electrode 22, and a state of equilibrium is reached where the number of secondary electrons lost by the strip is equal to the number of primaries gained. The potential at which this occurs constitutes the maximum limiting potential of the strip. When the cathode ray beam falls on the section IA of a strip of the contact row, secondary electrons are prevented from leaving said strip by the electrostatic field produced by the electrode 24, and the potential of the strip therefore falls due to the negative charging of the strip. If the cathode ray, ,beam remains in contact with said section I4, this process continues until the potential of the strip reaches a value near to the potential of the electrode 24, at which potential secondary electrons begin to pass from the strip to the electrode 24, and equilibrium is again established between the number of electrons reaching and the number leaving the strip. The potential at which this occurs constitutes the minimum limiting potential of the strip.

The electrode system adjacent to the contact row thus effects a division of the contact row into two sections, although said sections may both be of the same material, similar to the division effected in the construction described earlier by the different secondary emission characteristics of the two materials constituting the contact surfaces of the contact row. One fundamental difference should, however, be pointed out. When division of the contact row into sections is effected by an electrode system of the type described, if the cathode ray beam falls partly on one section and partly on the other, then, unless said cathode ray beam happens to fall in the position where the gain of electrons on one section is exactly equal to the loss from the other section, in which case no change of potential of the strip of the contact row occurs, the potential of the strip with which the cathode ray beam makes contact becomes charged to one or other of the limiting potentials according to the position of the cathode ray beam, and does not charge to an intermediate potential as when full modulation, as in the last described method the division of the contact row into sections is efiected by the differing secondary emission characteristics of the materials of which the contact row is composed. The action of the device does not, in the present case, depend in any way on the existence of critical potentials for the substances constituting the sections of the contact row, and the limiting potentials of the strips of the contact row are controlled entirely by the potentials of the electrodes 22 and 24, and hence may be adjusted to any required values within certain limits.

Modulation of the carrier may be efiected by modulating the current intensity of the cathode ray beam, the cathode ray beam being caused to oscillate continuously at carrier frequency from one section of the contact row to the other, preferably symmetrically, and preferably from a position lying just wholly on one section to a position lying just wholly on the other section.

Alternatively, modulation of the carrier may be produced by applying the modulating signals to electrostatic deflection plates. In this case carrier signals with pure A. C. modulation may be applied to a single pair of electrostatic defiection plates. At zero modulation the cathode ray beam then falls in the particular equilibrium position on the contact row where, by making contact partly with one section of the contact row and partly with the other section, it causes electrons to be lost by one section at the same rate as they are gained by the other section, so that the potential of the contact row remains steady. Modulation causes the beam to oscillate on either side of this position, the amplitude of oscillation being proportional to the modulation level of the carrier signals applied to the electrostatic defiection plates. At full modulation the beam oscillates between positions lying wholly on one section of the contact row and positions lying wholly on the other section. At intermediate levels of modulation the beam always falls partly on one section and partly on the other. The rate of alternate positive and negative charging of the contact row, and hence the amplitude of the potential variation produced on the contact row in each carrier cycle, is proportional to the deviation of the beam from the aforesaid equilibrium position assumed at zero modulation, 1. e. proportional to the amplitude of the modulation of the carrier signals applied to the deflection plates.

Modulation of the carrier may also be produced by applying carrier signals to one pair of electrostatic deflection plates and the modulating signals to another pair, but in this case the modulating signals must first be rectified. The cathode ray beam is made to oscillate continuously at constant amplitude across the contact row by the carrier signals. At zero modulation it falls at all times on one particular side of the aforesaid equilibrium position, so that the potential of the contact row assumes permanently one of its limiting potentials. The received signals applied to the appropriate deflection plates deflect the mean position of the cathode ray beam onone side of the mean position at zero modulation, so that during part of each carrier oscillation it falls partly on the other side of the equilibrium position, thereby applying a pulse of potential change to the contact row, the amplitude of said potential change being proportional to the extent of the deflection on said other side of said equilibrium position, i. e. to the amplitude of the modulation signals. At

of modulation, the beam oscillates symmetrically between the two sections .of the contact row.

Preferably the section of the contact row the limiting potential corresponding to which is the equilibrium potential at zero modulation, is the section providing the lower limiting potential, as the power output of the device is thereby kept as low as possible.

With the electrode system as' described, the effective positive charging current of the section l5 (Figure 2) of the contact row may be much greater than the beam current of the part of the cathode ray beam making contact therewith, due to the large secondary electron emission coefiicient which the substance forming the contact surface may .possess. The section l4 of the contact row, however, is charged directly by the cathode ray beam, so that the negative charging current is the beam current of the part of the cathode ray beam making contact therewith. It is no advantage for the effective charging current of one section of the contact row to exceed that of the other, so that, with the arrangement as described, full advantage cannot be taken of the high charging current available for the section l5.

In order to take advantage of this it is necessary to increase the efiective negative charging current of the section I4. This may be achieved with the aid of the additional electrode 25 shown in Figure 2. This electrode consists of or is coated with a substance of high secondary electron emission coefiicient, and is so disposed that a substantial part of the cathode ray beam falls thereon. Thus it may consist of metallic strips situated opposite the spaces in electrodes 22 and 24, as shown in Figure 2. Secondary electrons are emitted due to the impact of the cathode ray beam on the electrode 25. The emission from this electrode is-enhanced by the fact that, when said electrode is composed of strips as described, the incidence of the primary electrons on said electrode is very oblique. Said electrode 25 is kept at the same or at a more positive potential than the electrode 24, so that the electrostatic field produced by said electrode 24 forces the secondary electronsfrom the electrode 25 on to the section I4 of the contact row. This effectively multiplies the beam current of the cathode ray beam making contact with said section l4 of the contact row.

The electrodes 22, 24 and 25 preferably all consist of members in the form of wires or strips of metal running parallel to the direction of scan ning, as already described for the electrode 22 with reference to Figure 1.

A screening electrode 26 is preferably provided between the sections l4 andl5 of the contact row, to prevent electrons emitted from the section l5 from passing to the section l4. It should be kept at a potential low with respect to the potentials usually appertaining to the elements of the contact row, and may conveniently form an appendant part of the electrode 24.

The form of electrode system described with reference to Figure 2 illustrates clearly the principles involved in this part of the invention.

Each of the electrodes has a special function. Electrode 25 multiplies the eifective current intensity of the cathode ray beam, and prevents secondary electrons from leaving the section II of the contact row. Electrode 24 screens elec trode 25 from the potential of the electrode .22, thereby preventing the secondary electrons emitted by the electrode 25 from passing to the electrode 22. Electrode 22 screens the remainder of the electron discharge device from the potentials of the electrodes 24 and 25 and the contact row. Other electrode systems may be used to effect the same results. The functions of the electrodes 24 and 25 may also be performed by a single electrode. The electrode system described is selected to illustrate the principles involved, as being very suitable for explanatory purposes.

The secondary electron emission coefficient of all substances becomes less than unity if the velocity of the primary electrons is sufliciently small. The action of the device depends on the secondary emission coefficient of the substance constituting the section l of the contact row being always greater than unity. For this reason the potential of the elements of the contact row must never fall below a certain value relative to the potential of the cathode of the electron disi charge device. The minimum limiting potential of the contact row must therefore be chosen at a certain safe value positive with respect to the cathode, the necessary value usually lying between 30 and 200 volts, the lower values being obtained when substances of high secondary emission coefficient are employed.

In the forms of the device which have thus far been described, the amount of light which can be modulated by each element of the light modulating cell is limited by the amount of energy which can be supplied by the cathode ray beam to one strip of the contact row in the time which it takes to scan said strip. If that strip is one of many, as when it represents one picture element of a high definition television picture, that time is a small fraction of the time taken to scan all the strips of the contact row, and the energy thus supplied to each strip is accordingly restricted. Modified forms of the device will now be described whereby this restriction may be over come.

Figures 3 to 6 are diagrammatic sections of such modified forms of the device.

In Figures 3 to 6 there are shown a light modulating cell H, a row of members I2 which serve to excite oscillation producing members associated with the light modulating cell, a contact row l3, within the envelope ID of an electron discharge device, each of the members of which contact row is connected to one of the members l2, the contact row being divided into sections [4 and I5, and an electrode 22. In Figures 3 and 4, additional electrodes 24, 25 and 26 are shown. These components of the device are in general similar to the components similarly numbered in Figures 1 and 2, and perform essentially the same functions as those described with reference to Figures 1 and 2. Equally, they may assume any alternative form mentioned with reference to Figure 1 or 2, and the form shown for these components in Figures 3 and 4 may be employed in the form of the device shown in Figures 5 and 6, or vice versa. As in the case of the contact row shown in Figure 1 or 2, the strips [3 of the contact row shown in Figures 3 to 6 are actuated by a cathode ray beam which makes contact therewith. In contradistinction to the cathode ray beam employed in the forms of the device shown in Figures 1 and 2, however, the cathode ray beam employed in the forms of the device shown in Figures 3 to 6 does not scan the strips of the contact row, but is of sufficiently large dimensions to cover simultaneously all the strips of the contact row. The portion of. the

cathode ray beam which makes contact with each strip of the contact row is independently modulated to the appropriate intensity determined by the received electrical signals, and so conveys a corresponding signal to said strip of the contact row.

The cathode ray beam is produced by a cathode 30, associated with a control grid 21 and an anode 28, with appropriate means to form an electron image on the contact row I3, such as an electromagnetic focusing coil 29 shown in Figures 3 to 6. A carrier signal is applied to a pair of electrostatic deflection plates 3|, whose function and action is essentially the same as described with reference to the deflection plates 2| shown in Figure 1. In the form of the device shown in Figures 3 and 4, modulation of the cathode ray beam is effected by applying the modulating signals to the control grid 21; in the form of the device shown in Figures 5 and 6, these signals are applied to auxiliary electrostatic deflection plates 33, shown in Figures 5 and 6; the action in the two cases is similar to that described with reference to the corresponding units l1 and 23 shown in Figure 1. However, as the cathode ray beam now has to be modulated in sections, the unit to which the received signals are applied must itself be divided into corresponding sections, and the received electrical signals must be applied in turn to those sections. In Figure 6 the electrostatic deflection plates 33 are shown so divided. When the grid 21 is divided, division is carried out in a similar manner, as shown in Figure 4. Said grid may then conveniently be constructed in the form of a wire mesh, suitably divided.

When modulationv is effected by electrostatic deflection plates, said plates should be situated very close to the electrode system 30, 21, 28, which produces the cathode ray beam, for an electron image of the section of the cathode ray beam passing between said deflection plates must be formed on the contact row. Conveniently the deflection plates may constitute the anode 28 or the grid 21, or part of one of those electrodes, said anode or grid being appropriately constructed.

One of the plates 33 may be continuous, and be kept at a constant potential, modulation being effected by increasing or decreasing appropriately the potential of the sections of the other plate. Preferably, however, both plates are divided, modulation being effected by increasin the potential of any section of one plate appropriately and decreasing the potential of the corresponding section of the other plate by substantially the same amount, the mean potential of every section of the plates therefore remainin substantially constant. This preferred construction is shown in Figures 5 and 6.

The sections of the deflection plates 33, or the sections of the grid 21 where this is divided, are connected to electrically conductive strips constituting an auxiliary contact row 32, which is scanned by an auxiliary cathode ray beam, the necessary modulation level being thereby applied to the sections of the plates 33 or of the grid 21. When the plates 33 are both divided, the sections of one plate are connected to the corresponding strips of one section 34, and the sections of the other plate are connected to the corresponding strips of another section 35, of the contact row, as shown in Figure 5, the sections 34 and 35 being mutually insulated.

The cathode ray beam which actuates the contact row 32 is produced by a cathode 38, associated with a control grid 31 and an anode 38, and appropriate means, such as an electromagnetic focusing coil 39, for forming an electron image onthe strips of the contact row 32. The cathode ray'beam is made to scan the contact row by appropriate means, such as a saw-tooth oscillation applied to a pair of electrostatic deflection plates 45.

When the grid 21 is divided, or when only one of the plates 33 is divided, so that the contact row 32 consists of a single row of strips, the signals from the cathode ray beam may be conveyed to the strips of the contact row by direct charging or preferably by secondary emission, the secondary electrons being collected by an electrode 40 shown in Figures 3 and 4, and modulation of the cathode ray beam may be effected by applying the modulating signals either to the control grid 31 or to electrostatic deflection plates 46 shown in Figures 3 and 4. When both of the plates 33 are divided, so that the contact row 32 consists of a double row of strips as shown in Figure 5, then one set of strips must be charged to a more positive potential and the other set must be charged simultaneously to a more negative potential. This may be achieved by employing an electrode system 30, M, 32, 43, adjacent to the contact row. These electrodes are essentially similar in construction and action to the electrodes 22, 2t, 25, 26, shown in Figures 2 and 3, except that, as the sections 33 and 35 of the contact row are not connected and are actuated simultaneously by the cathode ray beam, the section 34 always becomes charged more negatively and the section 35 always becomes simultaneously charged more positively by the cathode ray beam. The electrodes 40, ii, 32, 33, are maintained at appropriate potentials. Modulation is achieved by applying the modulating signals to the control grid 31.

Each strip of the contact row 32 thus becomes charged to an appropriate modulation level in each scan of the cathode ray beam. This modulation level is retained after the cathode ray beam has passed on, and so the signal thereby conveyed to the corresponding strip of the contact row I 3 likewise persists after the auxiliary cathode ray beam has scanned said strip of the contact row 32. In this way the energy conveyed to any strip of the contact row l3 in each scan may be made much greater than it would be if said contact row l3 were scanned directly by a cathode ray beam, thereby allowing more light to be modulated by the light modulating device H.

The modulation level produced on any strip of the contact row 32 in one scan must become substantially zero by the time the cathode ray beam returns in the next scan. For this reason every strip of said contact row must be connected by a leakage resistance of appropriate value to a member kept at the potential corresponding to zero modulation of the contact row 32. Such a leakage resistance is shown at 44 in Figure 5 connecting the sections 34 and 35 of the contact row, the mid-point of said leakage resistance being connected to the anode 28 which constitutes the potential corresponding to zero modulation of the auxiliary contact row 32 when said contact row is connected to electrostatic deflection plates such as those shown at 33. A similar resistance is shown at 44 in Figures 3 and 4 connecting the strips of the contact row 32 to an additional electrode 41, which is maintained at the potential corresponding to zero modulotion of the auxiliary contact row 32 when said contact row is connected to the grid 21, i'. e. at the most'positive potential of said grid '21 at which current from the cathode 30 is reduced to zero.

The electrode system associated with the cathode 30 must be screened from the electrode systemv associated with the cathode 36. In Figures 5 and 6 the grid 21 is shown constructed of such a form that it itself constitutes the necessary screening system. In Figures 3 and 4 the electrode 41 is shown constructed of such a form as to constitute the necessary screening system. Other screens, not shown in the diagrams for the sake of clearness, may be necessary for this or other purposes, but their use does not affect the principleof the device, and they are not described in detail herein.

The disposition of the various electrodes shown in Figures 3 to 6 is represented purely diagrammatically, and in practice the disposition may be modified in certain respects. For example, the divided grid 21 andthe divided strips of the contact row 32 shown in Figures 3 and 4 may, in practice, most conveniently lie in the same plane, the end of each strip of the contact row being connected to the end of the corresponding section of the grid.

What we claim and desire to secure by Letters Patent is:

1. A light modulating device comprising means for actuating a light cell of the kind efi'ecting modulation of a light beam by reason of mechanical oscillations produced in a light transmitting medium and having means responsive to changes of electric potential for producing said oscillations, including an electrode having two sections exhibiting difierent secondary electron emitting properties, an electrical connection between said sections and the said oscillationproducing means, means for producing an electron beam and means for directing said electron beam to aifect predominantly each of the said sections alternately to impart to said oscillationproducing means an alternating potential.

2. A light modulating device comprising means for actuating a light cell of the kind efiecting modulation of a light beam by reason of a row of independent trains of mechanical oscillations produced in a light transmitting medium and having separate means responsive to changes of electric potential for producing each such independent train, including a row of electrodes, one electrode being connected to each of said separate means and each electrode having two sections exhibiting different secondary electron emitting properties, an electrical connection between said sections and the said separate means, means for producing an electron beam, means to cause said beam to aifect the electrodes of the row successively, and means for directing said electron beam to affect predominately each of the said sections alternately to impart to the said separate means an alternating potential.

3. A light modulating device comprising means for actuating a light cell of the kind effecting modulation of a light beam by reason of a row of independent trains of mechanical oscillations produced in a light transmitting medium and having separate means responsive to changes of electric potential for producing each such independent train, including means for producing an electron beam, a row of electrodes, means to cause said beam to scan the electrodes of said between each of said electrodes and one of said separate means, each electrode having two directly connected sections constituted by contact surfaces having different electron emissive characteristics disposed alternately in the direction in which the electron beam scans the rows of electrodes, whereby each of said separate means has imparted thereto an alternating potential during the scanning of said row of electrodes by the beam.

4. A light modulating device comprising means for actuating a light cell of the kind effecting modulation of a light beam by reason of a row of independent trains of mechanical oscillations produced in a light transmitting medium and having separate means responsive to changes oi 'electric potential for producing each such independent trains, including a row of electrodes, one electrode being connected to eachof said separate means and each electrode having two sections exhibiting different secondary electron emitting properties, an electrical connection between said sections and the said separate means, means for producing an electron beam which normally makes simultaneous contact with all of the said electrodes, means for directing said electron beam to afiect predominately each of the said sections alternately to impart to the said separate means an alternating potential, and means whereby portions of the beam making contact with successive electrodes are affected successively in response to the application of modulating signals to control the value of the alternating potential thus imparted to each of the separate means in turn.

5. A light modulating device comprising means for actuating a light cell of the kind effecting modulation of a light beam by reason of a row of independent trains of mechanical oscillations produced in a light transmitting medium and having separate means responsive to changes of electric potential for producing each such independent trains, including a row of electrodes, one electrode being connected to each of said separate means and each electrode having two sections ex-, hib iting different secondary electron emitting properties, an electrical connection between said sections and the said separate means, means for producing an electron beam which normally makes simultaneous contact with all of the said electrodes, means for directing said electron beam to affect predominately each of the said sections alternately to impart to the said separate means an alternating potential, control means whereby portions of the said beam making contact with successive electrodes are affected successively in response to modulating signals applied to said control means to control the value of the alternating potential thus imparted to each of said separate means in turn, means for producing an auxiliary electron beam, and means whereby said auxiliary electron beam serves to apply the modulating signals to said control means.

6. A light modulating device comprising means for actuating a light cell of the kind effecting modulation of a light beam by reason of a row of independent trains of mechanical oscillations produced in a light transmitting medium and having separate means responsive to changes of electric potential for producing each such independent train, including means for producing an electron beam, a row of electrodes, means to cause said beam to scan the electrodes of said row successively, an electrical connection between each of said electrodes and one of said separate means,

each electrode having-two directly connected sections constituted by contact surfaces having different electron emissive characteristics disposed in positions mutually adjacent in the direction at right angles to the direction in which the electron beam scans the row of electrodes, and means for directing said electron beam to affect predominately each of thesaid sections alternately during the scanning of the row of electrodes by the beam so as to impart an alternating potential to each of the said separate means in turn.

7. A light modulating device comprising means for actuating a light cell of the kind effecting modulation of a light beam by reason of mechanical oscillations produced in a light transmitting medium and having means responsive to changes "of electric potential for producing said oscillations, including an electrode having two sections, an electrical connection between said sections and the said oscillation-producing means, a system of electrode members arranged adjacent to one of said sections in order to modify the secondary electron emitting property of said section with respect to the secondary electron emitting property of the other section, means for producing an electron beam, and means for directing said electron beam to affect predominately each of the said sections alternately to thereby impart to said oscillation-producing means an alternating potential.

8. A light modulating device as claimed in claim '7, wherein said system of electrode members comprises means serving to prevent secondary electrons from leaving the section adjacent which the system is disposed.

9. A light modulating device as claimed in claim 7, wherein said system of electrode members comprises means serving to multiply by secondary emission the current intensity of the part of the electron beam which impinges on the section adjacent which the system is disposed.

10. A light modulating device as claimed in claim '7, wherein said system of electrode members comprises means serving to collect secondary electrons emitted from the section adjacent which the system is disposed.

11. A light modulating device comprising means for actuating a light cell of the kind efl'ecting modulation of a light beam by reason of a row of independent trains of mechanical oscillations produced in a light transmitting medium and having separate means responsive to changes of electric potential for producing each such independent train incIuding a row of electrodes, one electrode being connected to each of said separate means and each electrode having two sections, an electrical connection between said sections and the said separate means, means for producing an electron beam, means to cause said beam to affect the electrodes of the row successively, a system of electrode members arranged adjacent to one section of each electrode in order to modify the secondary electron emitting property of said section with respect to the secondary electron emitting property of the other section, said electrode members extending parallel to the direction in which the electron beam successively affects the electrodes forming the row, and means for directing said electron beam to afiect predominately each of the said sections alternately to thereby impart to the said separate means an alternating potential.-

PAUL NAGY. MARCUS JAMES GODDARD. 

